Fossils fuels a and more particular heavy cut fuels are the main cause of sulfur dioxide emissions. To this can be added the industries using or consuming sulfur or its mineral combinations, such as the industries for synthesis of sulfuric acid by combustion of sulfur, roasting of ores, calcining of certain mineral salts, metallurgy, etc. The atmospheric discharge of gaseous effluents containing sulfur dioxide constitutes a considerable pollution source and is responsible for serious damage to the environment.
Oxidation of sulfur dioxide by oxygen is slow, and numerous technologies for treatment of gaseous effluents are based on the absorption of sulfur dioxide by a solid base compound or a compound in the form of an aqueous suspension. The solid or liquid combination thus obtained is mechanically separated from the gaseous flow, then optionally treated. Under these conditions, it is still necessary to eliminate these unusable compounds in a way that it acceptable on the economical and ecological level.
A second mode of purification consists of oxidizing the sulfur dioxide by oxygen or air in an aqueous medium, which provides a more or less dilute sulfuric acid solution which can be used.
Certificate of addition FR.2.058.158 describes a process for purification of fumes comprising sulfur dioxide, consisting a first stage of a reaction with the ozone carrier by a carrying fluid such as oxygen and a second stage putting the fumes in contact with a sulfuric acid solution.
However, to achieve this oxidation with kinetics which are compatible with the contingencies of industrial operation, it is necessary to add catalysts to the oxidation medium, preferably metal salts, and particularly manganese salts.
From French patent 2,186,285 a process is known in which the gaseous effluent containing sulfur dioxide is contacted with a dilute sulfuric acid solution in the presence of manganese sulfate which has been previously oxidized by ozone.
Purification of gaseous effluents containing sulfur dioxide by oxidation catalytic reaction leading to a more or less concentrated sulfuric acid solution has been the object of numerous studies.
French patent 2,238,669 describes the oxidation of a gaseous effluent containing oxygen and sulfur dioxide by prolonged contact with a dilute sulfuric acid containing manganese sulfate.
Although this method theoretically makes possible the further use of the sulfur dioxide contained in the gaseous effluents, its practical use poses various problems. The amounts of catalysts used are great; moreover, the sulfuric acid collected is relatively dilute and contains a high level of impurities coming from the residual catalyst. Obtaining marketable sulfuric acid from such a process would require the use of one or more purification stages to eliminate the catalysts, and a concentration stage to achieve an acceptable acid strength. It is clear that these stages would greatly burden the overall economy of the process with debt.
French patent 2,323,632 discloses a process for treatment of gaseous effluents containing sulfur dioxide comprising putting said effluents in contact with an aqueous sulfuric acid solution in the presence of oxygen or air and a catalyst such as a manganese ore salt, according to which the aqueous sulfuric acid solution obtained after contact and containing sulfur dioxide is subjected to the action of a oxidation initiator of the ionizing radiation type. The sulfuric acid solution is irradiated either by a source of ionizing radiation coming from X-rays, gamma rays of radioactive isotopes as .sup.60 Co, .sup.137 Ca, and the breakage of a charge-particle accelerator. According to this process a degree of purification of gases greater than 0.95 can be attained, while obtaining a sulfuric acid in a concentration of up to 50%.
This process seems to offer only scientific laboratory interest, because its use on an industrial scale is completely inconceivable because of the various serious drawbacks. Treatment of large volumes of sulfuric acid by irradiation results in a requirement for apparatus at the purification site is vey expensive. Further, the use of X or gamma rays at an industrial site is dangerous and poses very serious safety problems for the personnel.
In connection with these observations and considering that the active species responsible for oxidation of the sulfur dioxide by oxygen is a manganese ion with a degree of oxidation greater than +II : Mn.sup.n+, an effort was made to find an economically acceptable industrial purification process, with a very high efficiency, greater than 99% and leading to obtaining a directly usable high-strength sulfuric acid solution without further treatment while using a minimal amount of catalyst.